1. Field of the Invention
The present invention relates to a submarine optical cable comprising a central polymeric buffer tube, said cable being particularly suitable for long-haul repeatered systems.
Submarine cables for long-haul repeatered systems are typically required to contain a relatively small number of optical fibers, the maximum number of optical fibers being determined by the capacity of optical amplifiers-repeaters. Accordingly, the fiber count for submarine optical cable for long-haul repeatered systems is typically from a minimum of 4 fibers up to a maximum of 48 fibers.
The total length of long-haul submarine cables (also called xe2x80x9csegmentsxe2x80x9d) is typically of about 1500-2000 km. These lengths are generally formed by a number of optical cable spans (50-80 Km length), generally separated from each other by amplifiers. The cable-spans are preferably joined together with the amplifiers in the manufacturing plant, in order to manufacture the final 1000-2000 Km length segments which are then loaded on the cable laying ship. The total length of repeatered submarine systems may vary from about 1000 to about 10000 Km, joining two or more of the above segments, if necessary.
2. Description of the Related Art
In view of the low fiber-count capacity requirements and of their relatively long length, these types of cables are thus typically of reduced dimensions, comprising a single central buffer tube carrying the optical fibers. The inner diameter of said fiber-containing buffer tube is generally lower than about 5 mm.
On the other side, fiber-count requirement for submarine cables for non-repeatered systems is generally higher (e.g. up to about 96 fibers) and thus the dimensions of said cable can be, if necessary, increased accordingly. The length of said cables is generally of about 50-150 Km, up to a maximum length of 400-500 km.
A number of cables designs for submarine installation is known in the art.
For instance, U.S. Pat. No. 5,125,062 discloses an undersea cable comprising a central metallic tube, filled with a sealing compound, e.g. silica gel, and containing optical fibers embedded therein, said tube being surrounded by a helical lay of metallic (preferably steel) wires. Interstices between wires and between the helical lay and the central tube are filled with a sealing material (preferably polyurethane resin) which opposes longitudinal propagation of water along the cable. Alternatively, the central tube can be made of plastic and in this case the helical lay also presents the characteristics of an arch for withstanding pressure.
U.S. Pat. No. 4,684,213 relates to a submarine cable comprising a pressure resistant steel tube containing optical fibers, surrounded by two layers of steel wires and by an outer metal tube made of copper or aluminum. Dams of a sticky compound and/or of a jelly of plastic material are disposed at regular intervals inside the central tube and in the gaps between the lay of wires disposed between the central tube and the outer tube.
U.S. Pat. No. 5,463,711 discloses an underwater cable for shallow-water, comprising a central tube made of metal, optical fibers arranged within said tube and surrounded by a water blocking material and six steel wires wound in a helical lay around the central tube.
In the article of D. Felicio, xe2x80x9cA non-repeatered achievementxe2x80x9d, Telcom report International 19 (1996), pp. 22-25, a submarine cable for non-repeatered systems is disclosed (MINISUB CT(copyright)), where the fibers are accommodated into an hermetically sealed central copper tube having an outer diameter of about 5 mm. Said cable is presented in this document as an evolution of a previous cable design (MINISUB 16C(copyright)) comprising a central plastic buffer tube surrounded in turn by a hermetic copper tube (6.1 mm diameter).
In the article of J. F. Libert et al., xe2x80x9cA New Undersea Cable For The Next Milleniumxe2x80x9d, Conference Proceedings Suboptic ""97, pp. 120-128, a deep-water light weight cable specifically designed for long haul repeatered systems is disclosed. Said cable comprises Large Effective Area (LEA) fibers housed within a jelly filled welded steel tube, which acts as a pressure-resistant component in the cable. The manufacturing process of the tube is controlled in order to provide a small excess length fiber (0.1%). As disclosed in said article, the combination of the slight excess fiber, jelly filling and pressure resistant tube provides a very low stress environment for the fibers, which minimize loss increments in the more bend-sensitive fibres that may be used in wavelength-division-multiplexing (WDM) and high bandwidth systems. As shown in said article, LEA fibers are much more sensitive to bending-induced loss than standard dispersion-shifted fibers (SDS fibers) currently in use.
In view of the above, it is thus apparent that current trends in the production of optical buffer tubes having relatively small dimensions, for use in submarine cables, are in the direction of using metal tubes.
Applicant has however observed that the production of metal buffer tubes of small dimensions (e.g. less than about 6 mm of outer diameter) may be somehow difficult, due to a series of drawbacks.
For instance, optical fibers can be subjected to an undesired over-heating during the welding of the metal tube, with the possibility of being damaged. For avoiding or at least limiting the effects of fiber overheating, a number of technical solutions are provided in the prior art, which solutions however introduce further operative steps or control techniques into the production process. For instance, U.S. Pat. No. 4,852,790 discloses to focalize a laser light above the surface of the tube, in order to weld and hermetically seal the abutted edges of the metal strip without excessive overheating of the underlying structure. U.S. Pat. No. 5,380,977 discloses the use of an optical fiber guide for guiding the optical fibers inside the metal tube, said guide being elastically urged on an inner wall opposite to the welded portion of the metal tube. U.S. Pat. No. 5,760,364 discloses a thermal diffuser interposed between the auxiliary tube for inserting optical fibers into the metal tube and the closure zone that is defined on said metal tube.
In addition, specific techniques and devices should be applied in order to achieve an optimal control on the excess fiber length value inside metallic tubes. For instance, U.S. Pat. No. 4,852,790 discloses the use of a gas flow inside the metal tube for blowing the optical fibers against the outer circumference of the metal tube as the metal tube wraps around the wheel. U.S. Pat. No. 5,231,260 discloses means for the control of the fiber excess length inside a metal tube, which comprises tension adjusting means for optical fiber and for the metal strip forming the metal tube and traction means, including tension variable means, for the metal tube. U.S. Pat. No. 5,143,274 discloses a method for manufacturing a metal tube containing a predetermined excess length of optical fibers, which comprises controlling the fiber speed, the temperature of the metal tube and the pull out tension of the tube.
Applicant has now observed that it would be advantageous to use polymeric materials for manufacturing such buffer tubes. However, Applicant has also observed that polymeric buffer tubes having reduced dimensions may pose additional problems, with respect to metallic tubes, as explained in the following.
In order to reduce the stresses on the optical fibers, it is preferable to lay the optical fibers inside the tube with substantial no excess length or with a slight excess (less than about 0.1%) with respect to the length of the buffer tube. If the fiber is disposed inside a buffer tube with an excess length, its path within the buffer tube will not be linear but the fiber will be disposed on a sinusoidal/helical path. The higher the fiber excess length, the more apparent the sinusoidal/helical path.
In this connection, Applicant has however observed that excess lengths higher than about 0.1% are incompatible with small inner diameter buffer tubes (typically less than about 3 mm for fiber count of 12÷24). In fact, if said excess length is too high (e.g. more than about 0.1%), the sinusoidal/helical path of the fiber will contact the walls of the buffer tube, with consequent problems of microbending and macrobending on the fiber. On the other side, buffer tubes with larger inner diameters may allow higher excess lengths, as the sinusoidal/helical path of the optical fiber within the buffer tube can be arranged within a larger cross-sectional area.
However, Applicant has observed that with the conventional polymer materials used to manufacture buffer tubes and with the conventional extrusion techniques, it is very difficult to have a sufficiently precise control over the selected excess fiber.
In particular, as observed by the Applicant, excess fibre length variations must be controlled both during the manufacturing stage of the buffer tube on the extrusion line and during the storage period of the buffer tube (the tube being typically wound on a storage reel at the end of the manufacturing process). Typically, storage periods (i.e. the period between the wounding of the tube on the reel and its use to make the cable) may vary from several hours to approximately one week.
Fibre excess length in polymeric buffer tubes is generally obtained by acting through the thermal properties of the polymeric materials forming the tube. In fact, the extruded tube, upon cooling, tends to substantially reduce its overall length due to the relatively high thermal coefficient of expansion of polymeric materials (e.g. from about 50xc3x9710xe2x88x926 1/xc2x0 C. to about 300xc3x9710xe2x88x926 1/xc2x0 C., depending on the specific material, the extrusion conditions, etc.). Thus if optical fibers are supplied without substantial tension to the extruder with the same speed of the extruded tube leaving the extruder, and if the material forming the buffer tube has a longitudinal contraction upon cooling of e.g. about 0.1%, the resulting excess fiber will be of about 0.1% at the end of the extrusion process. Said excess length may be further modified by unwinding the optical fibers from the pay-off bobbins with a predetermined tension. In this case the fiber will be housed into the buffer tube with a minor length than if it had been tensioned, thus obtaining a final excess length fiber lower than the one obtainable without applying said tension to the fibers. Anyhow, whichever manufacturing method is applied, it is generally possible with the known manufacturing technologies to obtain, at the end of the extrusion process, buffer tubes with relatively reduced deviations (+/xe2x88x920.02%) from a predetermined excess length value.
The Applicant has however observed that once the buffer tubes made according to known techniques are collected (e.g. on a reel), the polymeric material forming the tube tends to additionally settle and, in particular, to shrink. This settling generally cannot be foreseen; however, it usually causes additional tube shrinking leading to uncontrollable variations (usually an increase) of the set excess fibre values.
This shrinking during the storage period of the tubes, as observed by the Applicant, may in some cases result in variations of the excess length comparable to the predetermined excess length value, with the result of substantially modifying the final excess fibre value and creating problems in the subsequent use of the tube in making the optical cables.
In particular, the applicant has observed that, at high production speeds, the tube is typically wound on the reel in random crossed turns. This unorderly tube winding generates gaps randomly distributed on the tube skein collected on the reel. The tube may detensionate more easily near these gaps and shrink, while detensioning may be obstructed in other areas. This causes different, uncontrollable shrinking of the tubes wound on different reels and also along different lengths of the same tube wound on the same reel.
Thus, in case of a predetermined value of excess fiber length of e.g. about 0.1%, applicant has measured, after a storage period of about one week, a final excess length of from about 0.12% to about 0.3%, depending on the different materials, extrusion and storage conditions. While a certain variation of the value of excess fibre length with respect to the predetermined one value can be acceptable for buffer tubes having a relatively large internal diameter (e.g of about 10 mm or more), Applicant has observed that even small variations with respect to the predetermined value are however hardly acceptable for buffer tubes having a relatively small internal diameter (e.g. below 5-6 mm). As a matter of fact, if said variation is too large, it may cause the fiber to be disposed onto a path where the fiber is forced into contact with the inner wall of the buffer tube thus causing microbending effects onto the fiber. Applicant has noticed that this problem becomes much more relevant when LEA fibers are used, which fibers are more sensitive to microbending and macrobending phenomena. The term LEA fibers is intended to encompass within its meaning those optical fibers having a large effective area, in particular optical fibers having an effective area of at least 7 xcexcm2 or greater.
It goes without saying that by using a metal buffer tube, the predetermined value of excess fiber length remains substantially unaltered after the completion of the manufacturing process and during the subsequent storage of the buffer tube, as metal materials are substantially insensible to post-production shrinkage.
Applicant has now found that it is however possible to produce a submarine cable with a central polymeric buffer tube of reduced dimensions, wherein the excess fiber length can be controlled within a limited desired range with respect to a predetermined value of excess fiber length.
In particular, Applicant has found that such excess fiber length can be controlled by embedding a longitudinal reinforcing element within the walls of the polymeric tube. This reinforcing element should however be of relatively small diameter, in order to comply with the requirements of reduced dimensions of the buffer tube. Applicant has observed that, notwithstanding this relatively small diameter, said longitudinal reinforcing element allows for a good control over the longitudinal shrinkage of the polymeric buffer tube within the submarine cable.
Applicant has furthermore observed that for such buffer tube of relatively small internal diameter (e.g. below about 3 mm), it is rather difficult to achieve a complete filling of said tube with a suitable water-blocking gel. As a matter of fact, because of the relatively small diameter of buffer tubes specifically adapted for submarine cables and of the relatively high viscosity of the filling jelly compound (generally from about 50 Paxc2x7s to about 220 Paxc2x7s), the filling of the tube can only be completed for about 80-95% of the internal volume of the tube. In addition, an incomplete filling of the buffer tube may be desirable in some instances, also for buffer tubes of larger dimensions. For instance, as disclosed in European Patent Application Publ. No. EP 883007, if optical fibers are exposed to hydrogen gas, the transmission properties thereof are altered, the higher being the partial pressure of hydrogen the more relevant said alterations. EP 883007 thus suggests filling only partially the buffer tube with a filling material (maximum 95%), in order to leave an expansion volume inside the tube so that the partial pressure of hydrogen is kept relatively low.
Typically, from about 5 to about 20% of the internal volume of the buffer tubes can be left free from water blocking gel material.
Applicant has observed that, while this incomplete filling can be considered of relatively low importance for terrestrial cables (where longitudinal water penetration tests are conducted under a water head of 1 m), it becomes much more important for underwater cables. As a matter of fact, Applicant has observed that the voids due to the incomplete filling tend to dispose longitudinally along the whole length of the buffer tube, thus creating a preferential path along which a high pressure water flow (e.g. of about 500 bars if the cable is laid at a depth of about 5000 m) can flow with relatively low head losses for a relevant length of the cable before being blocked.
In addition, if water has a preferential flow path inside the buffer tubes, the respective hydrostatic pressure may act on the inner surface of the plastic tube, thus increasing the diameter of the same; as a consequence, the flow path is increased in dimension and the water flow increases the speed and the penetration length inside the buffer tube.
Applicant has now found that by providing a suitably dimensioned longitudinal cavity (or xe2x80x9cgapxe2x80x9d) defined along the outer surface of the above mentioned polymeric buffer tube containing optical fibers, it is possible to substantially reduce the longitudinal flow of water inside said buffer tube. In particular, the Applicant has observed that the dimensions of the longitudinal space surrounding the buffer tube should be selected in order to provide along said space a preferential flow path for the water accidentally penetrated inside the cable, with respect to the flow path inside the buffer tube, thus preventing water from flowing inside said buffer tube. In other terms, the head losses of the water flowing along said longitudinal space surrounding the buffer tube should be lower than the head losses of the water flowing inside said buffer tube. As a consequence, the hydrostatic pressure of the water penetrated into the above gap acts onto the outer surface of the polymeric buffer tube, thus allowing an effective compression of said buffer tube with a consequent reduction of the cross-sectional area of the tube and a corresponding further increase of the head losses inside said buffer tube.
Thus, a submarine cable according to the present invention comprising a buffer tube having at least one reinforcing element embedded in the walls thereof allows both an effective control over the predetermined excess fiber length inside said tube and over the longitudinal water penetration along said tube.
According to one aspect of the present invention, a submarine cable is provided comprising:
a polymeric buffer tube, defined by peripheral walls and containing at least one tubular passage, said tubular passage containing at least one optical fiber loosely housed therein;
a substantially non-deformable and hermetic elongated hollow body defined by an inner and an outer surface, said inner surface being disposed to longitudinally surround said polymeric buffer tube;
wherein at least one reinforcing element longitudinally extending along the whole length of the tube is embedded in the peripheral wall of the of said buffer tube.
In order to control the excess fiber length and, particularly, the post-extrusion shrinkage of the buffer tube, said reinforcing element preferably has a longitudinal stiffness higher than that of the polymeric buffer tube, preferably at least four times greater than that of the polymeric buffer tube. The longitudinal stiffness of an elongated element is defined as the product of the cross section area of said element times the Young modulus of the material forming said element.
According to a preferred embodiment, the material forming said reinforcing element has an elastic limit of at least 0.2%, preferably up to about 0.5%.
Typically, said reinforcing element has a substantially circular cross-section, with a diameter of from about 0.3 mm to about 0.8 mm, preferably of from about 0.4 mm to about 0.6 mm. Preferably, said reinforcing element is a metal wire, in particular a steel wire.
According to a preferred embodiment said tubular passage has a diameter not greater than about 3 mm, preferably not greater than about 2 mm. The outer diameter of the buffer tube can advantageously be less than about 6 mm, preferably less than about 5 mm. The thickness of the walls surrounding the tubular passage can vary from about 0.2 mm to about 0.5 mm.
According to a preferred embodiment, said substantially non-deformable and hermetic elongated hollow body comprises:
an inner layer comprising at least one annular lay of metal wires, e.g. of steel, preferably wound as a helical lay around the buffer tube, said helical lay presenting the characteristics of an arch for withstanding the relevant hydrostatic pressure at the selected depth; and
an outer layer comprising a metal tube, e.g. of copper or aluminum, conferring the hermetic protection (from possible radial penetration of water or gases) to the inner structure of the cable.
Preferably, said metal tube is formed onto said annular lay of metal wires.
Alternatively, said substantially non-deformable and hermetic elongated hollow body surrounding the tube can be a metal tube, made of e.g. copper, steel or aluminum, having a predetermined thickness, elastic modulus and yield strength so as to resist to a selected hydrostatic pressure. Such a metallic tube provides an hermetic protection towards radial penetration of water or gases into the inner structure of the cable where the optical core is located and can be used, in particular if it is made of copper, for supplying electric power to the system components.
According to a preferred embodiment, a longitudinal cavity is defined between the outer surface of the polymeric buffer tube and the inner surface of said non-deformable elongated hollow body, said cavity being at least partially filled with a water-blocking element and having a size such that, in case of accidental ingress of water inside said cable:
a water flow through said polymeric buffer tube is caused to have a first head loss;
a water flow through said longitudinal cavity is caused to have a second head loss;
said second head loss being lower than said first head loss.
Advantageously, the head losses of the water flow along said longitudinal cavity can be lower than about 75% of the head losses of the water flow inside the polymeric buffer tube, preferably lower than about 50%, and more preferably lower than about 30%.
The polymeric material forming the buffer tube can be advantageously selected from the group consisting of polyester (e.g. polybutyleneterephtalate), polyolefin (e.g. polyethylene, polypropylene, copolymer ethylene-propylene), or polyamide. The tube may comprise one or more layer of said polymeric materials. The polymeric material forming the tube will have a predetermined thickness and elastic modulus so that when a hydrostatic pressure of a selected degree is applied on the outer surface of said tube (as a consequence of the water penetration at the relevant laying depth of the cable), the tube is sufficiently compressed so as to reduce the void volume inside the mass of the filling material, thus reducing the distance covered by the water flow. Advantageously, the compression of said tube should be such as to reduce its original cross-sectional area of at least about 3%, preferably of at least 5%, when subjected to the relevant hydrostatic pressure.
The water-blocking element disposed within the longitudinal cavity along the polymeric buffer tube should be such as to allow the water penetrating in said longitudinal cavity to flow with less head losses than inside the polymeric buffer tube at the initial stage of the penetration, while allowing said head losses to gradually increase in time, eventually blocking the flow of water in said longitudinal cavity within a predetermined length of the cable from the inlet point of the water.
In general, the block of water is intended to be effective if water penetrates inside the cable for a distance of less than 1000 meters in two weeks at a pressure of 550 bar.
Preferably, said water blocking element is a filling material, e.g. an elastomer such as polyurethane or a jelly-like composition, discontinuously disposed along the longitudinal direction of said longitudinal passage.
Alternatively, said water blocking element is a water blocking tape which is wrapped around the buffer tube.